Handover (HO) refers to an operation in which a terminal moves from the wireless interface of a base station to the wireless interface of another base station. Hereinafter, a handover procedure in a typical IEEE 802.16e system will be described.
In an IEEE 802.16e network, a serving base station (SBS) may broadcast neighbor base station information through a neighbor advertisement (MOB_NBR-ADV) message to notify information (topology) on a basic network configuration to a mobile station (hereinafter, referred to as a “terminal”).
The MOB_NBR-ADV message may include system information on a serving base station and neighbor base stations, for example, preamble index, frequency, handover (HD) optimization capability, downlink channel descriptor (DCD)/uplink channel descriptor (UCD), and the like.
the DCD/UCD may include information that the terminal should know to perform information correspondence through uplink and downlink. For example, there are information such as handover (HD) trigger, medium access control (MAC) version of a base station, and media independent handover (MIH) capability.
More specifically, DCD includes downlink burst profiles (DL_Burst_Profiles), which are information for allowing a terminal to decode messages transmitted by a base station. All terminals within the coverage of the relevant base station are preferably able to receive messages broadcasted by the base station, and thus a DL_Burst_Profile setting value that is most robust to an error will be applied, and therefore, it rarely occurs when the setting value is changed. However, a message transmitted in unicast to each terminal sends a suitable setting value adapted to the channel state of a terminal, and thus the DL_Burst_Profile setting value during unicast may be different for each terminal. UCD includes UL_Burst_Profiles, which are information for allowing a terminal to send messages to the relevant base station.
As described above, system information in a typical IEEE 802.16e system may be transferred through UCD in uplink and DCD in downlink, and may have a maximum transmission period of 10 seconds. At this time, when system information is updated, the base station does not allow the system information of the relevant base station possessed by a terminal through scheduling to be mismatched so that all terminals may recognize the updated system information at least once. For this purpose, in case of UCD, the period of a UCD transition interval start and a UCD transition interval expired is defined, and the base station is supposed to transmit new UCD at that period.
A procedure in which the terminal that has acquired the system information of the neighbor base station through the foregoing method performs handover in an IEEE 802.16e network will be described in more detail.
A handover procedure in a typical IEEE 802.16e network may include three processes, such as handover (HO) initiation & preparation, handover (HO) execution, and handover (HO) completion.
An example of the basic handover procedure having the foregoing configuration will be described with reference to FIG. 1.
FIG. 1 illustrates an example of a handover procedure that can be carried out in an IEEE 802.16e system.
Referring to FIG. 1, first, a mobile station (MS) may be connected to a serving base station (SBS) to perform data exchange (S101).
The serving base station may periodically broadcast information on a neighbor base station in which the serving base station itself is located (S102).
While corresponding with the serving base station, the terminal may start to scan candidate base stations (candidate HO BSs) using a handover (HO) trigger condition. Under a handover condition, for example, when exceeding a predetermined hysteresis margin value, the terminal may transmit a handover request (MOB_MSHO-REQ) message to the serving base station to make a request for performing a handover procedure (S103).
The serving base station may notify a handover request of the terminal to the candidate base stations (candidate HO BSs) included in the MOB_MSHO-REQ message through a HO-REQ message (S104).
The candidate base stations (candidate HO BSs) may take preliminary measures for the terminal that has requested handover and transfer information associated with handover to the serving base station through a HO-RSP message (S105).
The serving base station may transfer information associated with handover that has been acquired through the HO-RSP message from the candidate base stations to the terminal through a handover response (MOB_BSHO-RSP) message. Here, the MOB_BSHO-RSP message may include information for performing handover, such as an action time for handover, a handover identifier (HO-ID), a dedicated handover (HO) CDMA ranging code, and the like (S106).
The terminal may determine one target base station among the candidate base stations based on information included in the MOB_BSHO-RSP message received from the serving base station. Accordingly, the terminal may transmit a CDMA code to the determined target base station to attempt ranging (S107).
The target base station that has received a CDMA code may transmit the success or failure of the ranging and physical compensation values through a ranging response (RNG-RSP) message to the terminal (S108).
Next, the terminal may transmit a ranging request (RNG-REQ) message for authentication to the target base station (S109).
The target base station that has received the terminal's ranging request message may provide system-2 information that can be used in the relevant base station, such as a connection identifier (CID), to the terminal through a ranging response message (S110).
If the target base station has successfully completed the authentication of a terminal and sent all update information, then the success or failure of handover may be notified to the serving base station of the terminal through a handover complete message (HO-CMPT) (S111).
Subsequently, the terminal may perform information exchange with the target base station that has performed handover (S112).
In an IEEE 802.16m system, there is a modification in the name and/or function of each medium access control (MAC) management in the foregoing handover procedure.
The handover procedure that can be carried out in an IEEE 802.16m system is similar to the handover procedure in an IEEE 802.16e system as described above. However, there is a modification in the name and/or function of each medium access control (MAC) management as follows.
MOB_NBR-ADV AAI_NBR-ADV: The relevant message includes system information transferred not in a DCD/UCD form but in an S-SFH form.
MSHO-REQ→AAI_HO-REQ
BSHO-RSP→AAI_HO-CMD
RNG-REQ (CDMA code)→Ranging preamble code
RNG-RSP (ranging status)→AAI_RNG-ACK (ranging status)
RNG-REQ (MAC message)→AAI_RNG-[0031] REQ
RNG-RSP→AAI_RNG-RSP: The relevant message includes TSTID or ATID, which is a station identifier, instead of CID.
Furthermore, in an IEEE 802.16m system, the system information of a base station is transmitted through a superframe head.
Hereinafter, the frame structure and superframe header of an IEEE 802.16m system will be described.
FIG. 2 illustrates an example of a physical frame structure used in a wireless metropolitan area network (MAN) mobile communication system on the basis of an IEEE 802.16 system.
Referring to FIG. 2, a superframe may have a length of 20 ms, and may include four frames.
One frame may include eight subframes again, and the eight subframes may be classified into a downlink subframe region and an uplink subframe region including a predetermined number of subframes according to a ratio of downlink and uplink (DL/UL ratio). In case where the uplink/downlink (UL/DL) ratio is 5:3 as illustrated in FIG. 2, five of the eight subframes are allocated to downlink subframes (SF0 through SF4), and the remaining three are allocated to uplink subframes (SF5 through SF7).
Between the downlink subframe region and the uplink subframe region, there exists an idle time, namely, a transmit/receive transition gap (TTG) to which data symbols (i.e., valid symbols) including data are not allocated. Furthermore, there may also exist an idle time, namely, a transmit/receive transition gap (TUG) subsequent to the downlink subframe region. Furthermore, one subframe may include six OFDM symbols again.
Using the foregoing frame structure, the base station and terminal may perform data exchange. For example, the terminal may receive data from the base station through a downlink subframe and transmit data to the base station through an uplink subframe. Furthermore, the base station may transmit data to the terminal through a downlink subframe and receive data from the terminal through an uplink subframe.
On the other hand, in the foregoing frame structure, a superframe header may be transmitted to the terminal through a first subframe of the superframe. The subframe header may include resource allocation information, system information, or the like in the frame or subframe unit included in the superframe header.
More specifically, in an IEEE 802.16m system, a superframe header (hereinafter, referred to as “SFH”) may include essential system parameters, system configuration information, and the like.
The superframe header may be divided into a primary superframe header (P-SFH) and a secondary superframe header (S-SFH). P-SFH transmitted for each superframe may include the least significant 4 bits (4 bit-LSB) information of the superframe number and information in association with S-SFH transmitted from the relevant superframe. The S-SFH transfers actual system information, and the system information is divided into subpackets according to its properties, which are referred to as S-SFH SPn (n=1, 2, 3). Each S-SFH SP IE is transmitted with different transmission periods (TSP1<TSP2<TSP3), respectively. Here, the system information are setting values according to the communication environment, such as ranging, power control, and the like, which are required for the terminal to perform downlink/uplink (DL/UL) transmission.
The information associated with S-SFH included in P-SFH may include an S-SFH change count indicating an S-SFH version currently being transmitted, an S-SFH scheduling information bit-map indicating whether S-SFH is transmitted in the relevant superframe, an S-SFH size indicating the number of logical resource units (LRUs) allocated for S-SFH transmission, an S-SFH number of repetitions indicating the transmission format of S-SFH, an S-SFH subpacket (SP) change bitmap indicating which S-SFH SP has been changed, and the like. At this time, the size of the S-SFH scheduling information bit-map and S-SFH subpacket (SP) change bitmap field is same as a total SP number of SSFH included in the relevant superframe.
When the terminal wants to perform handover to a specific target base station, it may cause a difference in the handover delay time according to whether or not the superframe header (SFH) information of the target base station possessed by the terminal is the latest. If handover is carried out in a state that SFH of the target base station has been received through a neighbor advertisement (AAI_NBR-ADV) message, then the target base station may perform network re-entry to complete handover.
However, the terminal may not have received a neighbor advertisement (AAI_NBR-ADV) message including SFH of the target base station, or may receive an AAI_NBR-ADV message in which a change of the SFH of the target base station has not yet been reflected. The terminal performing handover in this state receives all SFHs from the target base station and then performs network re-entry. In other words, it means that the handover delay time increases as much as the time required for the terminal to receive all S-SFH SP1/2/3, and additionally, the terminal cannot perform a handover (HO) optimized procedure such as dedicate ranging or seamless handover (HO) using a dedicated ranging code.
In other words, if the terminal does not receive any part of target base station SFH or receives SFH directly from the target base station while performing handover in a state of not having the latest information, the handover delay time may be greatly increased. Due to this, the minimum handover delay time defined by the standard may not be satisfied.